Biosynthesis of Fusarium culmorum trichothecenes. The roles of isotrichodermin and 12,13-epoxytrichothec-9-ene

Two different approaches enabled us to unambiguously establish the intermediacy of isotrichodermin and 12,13-epoxytrichothec-9-ene in the biosynthesis of the trichothecenes 3-acetyldeoxynivalenol and sambucinol, respectively. The kinetic pulse-labeling method enabled us to detect a plausible precursor to 3-acetyldeoxynivalenol biosynthesis. Feeding experiments using the pure 14C-labeled intermediate established that it was incorporated 27% into 3-acetyldeoxynivalenol but not to sambucinol. The 14C-precursor was subsequently used as a marker to purify the unlabeled intermediate which was shown to be isotrichodermin by spectroscopic techniques. In order to trace the enriched carbons incorporated into 3-acetylde-oxynivalenol, specifically deuteriated isotrichodermin was synthesized and fed to Fusarium culmorum. The 2H NMR spectrum of the derived 3-acetyldeoxynivalenol proved conclusively the position of the deuteriums and that isotrichodermin is a major biosynthetic precursor. The proof that isotrichodermin is converted in vivo to 3-acetyldeoxynivalenol but not to sambucinol led us to postulate that 12,13-epoxytrichothec-9-ene might have an important role in the biosynthesis of trichothecenes. We synthesized 12,13-epoxytrichothec-9-ene with tritium at C-15 or with two deuteriums at C-4 and two deuteriums at C-15. These labeled compounds enabled us to prove that 12,13-epoxytrichothec-9-ene is a major precursor to sambucinol biosynthesis but is neither converted to isotrichodermin nor to 3-acetyldeoxynivalenol. We also succeeded in isolating a biosynthetic intermediate between 12,13-epoxytrichothec-9-ene and sambucinol and characterized its structure as 3-deoxysambucinol by spectroscopic techniques (1H NMR, 2H NMR, 13C NMR, correlation spectroscopy, two-dimensional heteronuclear correlation experiments, and mass spectroscopy).